IOSR Journal of Electrical and Electronics Engineering (IOSRJEEE)
ISSN: 2278-1676 Volume 1, Issue 4 (July-Aug. 2012), PP 39-45
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
Y. S. Ravikumar
Research scholar, faculty of TE., SIT., Tumkur and Research Scholar of Dr. MGR University, Chennai-95
Abstract: Silicon carbide (SiC) is the perfect cross between silicon and diamond. The crystal lattice of SiC is
identical to silicon and diamond, but, exactly half the lattice sites are occupied by silicon atoms and half by
carbon atoms. Like-diamond SiC has electronic properties superior to silicon, but, unlike diamond it is also
manufacturable. The thermal leakage current (dark current) in SiC is sixteen orders-of magnitude lower as well.
As temperature increases, the leakage current increases, but, the temperature where the leakage current would
disrupt circuit operation is over 1000 °C in SiC, compared to about 250 °C in silicon. The SiC electronic
revolution began in the early 1990's when single-crystal wafers became commercially available for the first
time. During the intervening years, many different electronic devices have been demonstrated in SiC, with
performance often exceeding the theoretical limits of silicon. These include pin diodes, MOS field-effect
transistors (MOSFETs), metal-semiconductor field-effect transistors (MESFETs), and bipolar transistors
(BJTs), as well as specialized devices such as CCD imagers, Schottky diodes, static induction transistors (SITS)
and impact-ionization- avalanche-transit-time (MATT) microwave oscillators. These early digital logic gates
and linear elements are based on n-channel MOS technology, but, quickly followed by more sophisticated
CMOS integrated circuits.
Key words: Silicon Schottky diode, Silicon Carbide Schottky diode, MOSFET
I.
Introduction
Semiconductor power devices, especially diodes play important role in switching response. Low power
dissipation on the switching devices will give rise to highly efficient power electronic system. For example, if a
semiconductor device operates in a linear mode such as in power amplifiers of in linear regulators, it is expected
that some amount of energy will be lost in the power circuit before its energy reaches the output. High
efficiency of power converter requires a minimal loss of this energy from source to load. One of the energy can
be easily dissipated in the diodes in terms of heat leading to a lower efficiency of the converter [1].
The unipolar Silicon Schottky (Si) and Silicon carbide Schottky (SiC) diodes are commonly used in
power converters circuits. In spite of both diodes come from the same unipolar family, the issues of higher
switching losses with regards to reverse recovery losses have yet been solved. Nevertheless, the new SiC diode
has emerged in the market in recent years where they are expected to improve the efficiency of the converter by
allowing a further reduction in reverse recovery energy losses and hence increasing the performance. The
additional substance of carbide element in the power Schottky diode may eventually lower the reverse charge
current and thus, improve the overall transient response in the converter. An ideal semiconductor device would
inhibit large breakdown voltage, low voltage drop in the on-state, high switching speed and low power loss.
However in order to increase the performance of a semiconductor device, several additional doping
enhancements will be added during the fabrication stage, where the characteristics of the device will altered by
adding some impurity atoms to the pure semiconductor material. Today’s technology requires extensive
research to develop more powerful devices, not only to have lower switching losses, higher efficiency but also
improve reliability. For this reason, power losses in the device must be put into consideration. The main
contribution in this work is to verify that SiC high power diode having larger energy bandgap can indeed
produce better results in DC-DC converter. The analyses are mainly done using circuit simulator.
II.
Silicon Schottky Diode Family
The Silicon Schottky diode of Schottky barrier diode is widely used as a mixer or detector diode. In
addition to its low forward voltage drop, it may lead to lower levels of power losses in the circuit [2]. Since Si
diode is a unipolar device, the current transport is mainly due to majority carriers and therefore the speed is
faster. In addition, it has low turn-on voltage, high frequency capability and low capacitive effect [3]. SiC diode
is a wide-bandgap (WBG) semiconductor family from III-V group. It has the advantages in faster recovery
rimes as well as no dependence on temperature [4] and has the potential to operate more efficiently. In Si diode
however, the temperature may rise due to the increasing in electron’s thermal energy leading to higher peak
reverse recovery current [5]. This eventually gives higher power loss in the device.
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
In addition to smaller package and higher weight, SiC diodes also have higher critical field and barrier
heights compared to Si diode. This results in reduced on-state resistance and lower leakage current or SiC diode
[6]. It also has been demonstrated that the SiC diode has the potential to improve power FET performance [7].
Its energy bandgap is three times higher than Si and ten times higher in electrical breakdown strength.
Therefore, SiC can operate at operating voltage of 20 times with current densities up to 400 times higher than Si
diode [8]. SiC Schottky diode has a small forward voltage and the reverse breakdown voltage cannot be made
too high (currently about 200 V). It is used for rectification of power supplies for low voltage and high current
applications or in high frequency systems due to its small reverse recovery time.
Fig.1 The tetragonal bonding of a carbon atom with the four nearest silicon neighbors. The distance a and C-SiC
are approximately 3.08Ǻ and 1.89 Ǻ respectively.
Fig. 1 shows four Si atoms made a covalent bonding with a single Carbon © atom to form a SiC. The C
atom is located in the middle of the structure and the distances between all atoms which marked C-Si are equal.
The SiC possess increase tolerance to radiation damage, marking it a material suitable for defense and aerospace
applications. Due to high tolerance of temperature in SiC, it is chosen in various industries, such as aircraft,
automotive communications, power and power spacecraft.
Fig. 2 Energy Band Diagram of a Semiconductor.
The characteristics of SiC diode as a wide bandgap semiconductor device results in a more energy to
excite the electron from its covalent bond during turn-off compared to Si. Referring to Fig. 2, the wide bandgap
is measured from the distance between the conduction band and valance band. An insulator would have a larger
bandgap that would take lots of energy for the electrons to travel from the valence to conduction band while a
conductor would have no forbidden band. The energy is calculated from the difference between both bands. The
wider the bandgap, more thermal energy is required to excite the electrons enabling the device to operate at
higher temperature without affecting its electrical properties.
III.
Reverse Recovery
Reverse recovery is one of the properties in a diode. It can be a factor in determining the efficiency of
the applications. When a diode has been conducting in a forward bias long enough for it to establish steady,
there will be charges due to the presence of minority charge carriers. This charge must be removed to block in
reverse direction.
a. Soft recovery
b.Abrupt recovery
Fig. 3 Reverse Recovery Current Characteristic
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
The characteristics of reverse recovery current experienced by a diode is represented Fig. 3 above, t rr
represents the reverse recovery time, Irr is the peak reverse current whilst ta is the transition time due to charge
stored in depletion region of the p-n junction and tb is the time for the current to relax to zero. The peak reverse
recovery current depends on the falling rate of change in current during turn-off. In SiC diode, there will be less
or none reverse recovery current due to its ability to immediately remove stored charge. However, there are
differences observed during the recovery from the peak values. This is merely reflected from different device’s
fabrication techniques.
Normally, in SiC, the rising current rate takes a longer time (t rr ) as shown in part (a) in Fig 3. This
eventually reduces the turn-off speed. In other SiC type, The speed can also be slightly faster due to smaller t b
but with the cost of higher dissipation. This can be seen in part (b) as oscillation exists during the end stage of
turn-off time. In addition, if the falling current rate during the beginning of turn-off time is high as in the case of
non-schottky diode, the reverse current would also be high, leading to both high power dissipation and lower in
turn-off speed.
IV.
Diode characteristics
4.1 Static Characteristics
The I-V and reverse current are among the static characteristics of the device. Due to higher level of
majority carrier injection in Si diode, this causes a lower voltage drop and hence smaller capacitance to bias the
junction for turn-on process. This is the only advantages of Si diode compared to SiC. Here, SiC diode requires
a higher voltage to forward bias the device. Apart from that, SiC diode can handle larger reverse voltage as
compared to Si.
4.2 Dynamic Characteristics
The characteristic that changes with time is inherited in both devices. Si and SiC diodes are compared
in terms of the reverse recovery time, reverse recovery current and corresponding switching losses. The
comparisons in dynamic characteristics between two devices are tabulated in Table I. The SiC and Si diodes
used are of part number UPSC600 and B530C respectively.
Table I. Comparison of Dynamic Characteristics
Characteristics
Reverse Recovery Time
Reverse Recovery Current
Switching Losses
SiC Schottky (UPSC600)
Time Unchanged with
temperature variation
Negligible
Low
Si Schottky (B530C)
Increases as temperature increases
Increases as temperature increases
Slightly higher
Table I shows that SiC diode has advantages in all dynamic characteristics. Si diode suffers from
higher reverse recovery current and switching losses. This clearly indicates that additional carbide substance in
the device may improve switching speed and reduce power dissipation.
V.
Ac-Dc-Dc Converter (Chopper)
5.1 Simulation of DC-DC converter
Fig.4 shows the circuit diagram of AC-DC-DC converter, the LT spice simulation package is used for
simulating this circuit. Table 2 give the properties of the active deviced in this ciruit simulation.The input 230
V,50 Hz 1 sine wave is with Rser resistance 1Ω. The 1:1 power transformer selected to provide isolation in
order to have competability with main pwer suplly. Using MUR460 power diodes (D1-D4), bridge rectifier is
constructed and capacitor C2 is connected across bridge rectifier output reduce the ac component. The switching
device M1 (IXFX90N30) MOSFET used as switch. The gate pulse of amplitude 20 V with 25 µsec.on period of
50 µsec. total period is applied. Gate pulse Vg1 is shown in fig.5. The load for this circuit is used as inductive
load with an inductance of 50 µH and 55 . The laod current for this circuit design is 275 mA with 50 µsec
duration and it is shown in fig.6. The output voltage across the R2 and L3 is show in fig.7 and fig. 8. shows the
zoomed output at transition. The drain to source voltage of M1 is 108 volts to 228 volts which is shown in fig.9.
The gate to source voltageof M1 is 0.5 volts to 2.5 volts which is shown in fig.10. The current through schottky
diode D5 is 350 mA and switching loss of M1 is 235 W which are shown in fig.11 and fig.12.respectively.
Fig.13 shows the switching loss of the schottky dioce D5 and is 40mW. The gate current of M1 is shown in
fig.14 and is +1A during switch on and – 1A during turn off. The input to the ciruit is shown in fig. 15 nad
rectified voltage at across capacitor shown ing fig.16..
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
Fig.4 Circuit diagram AC-DC-DC converter
Table 2 Properties of the active devices used in the AC-DC-DC converter
Sl.No.
Recifier
MOSFET
Converter
Diode
IXFX()N30
Diode
MUR640
UPSC600
Manufactur
GI
IXYS
Microsemi
er
Polarity
Silicon
N-chan
Silcon
Carbide
Schottky
Vds[V]
300
0.033
Rds[on][]
Qgate[C]
Average
Current[A]
Breakdown
Voltage[V]
3.6e-007
4
1
600
600
Fig. Fig.5. Gate Pulse Vg1
Fig. 6. Load Current
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
Fig. 7. Output Voltage (Vo)
Fig. 8. Vo (zoomed near transition)
Fig. 9. Voltage Drain – Source
Fig. 10. Voltage Gate - Source
Fig. 11. Diode Current
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
Fig.12 MOSFET Switching losses
Fig. 13. Diode Switching Losses
Fig. 14. Gate Current
Fig.15. AC input voltage
Fig.16. Rectified Voltage Across Filter
5.2 RESULTS AND DISCUSSIONS
Correct choice of diode is important in understanding its effects on the overall performance in AC-DCDC converter. One of the effects is the voltage overshoot in the switch. Gate and drain voltages are prone to
stress when diode suffers from high reverse recovery and hence may give rise to less switching speed and high
dissipation in the circuit. Fig. 5 shows the forward overshoot in Vgs of M1 for SiC. Fig. 6 shows the load current
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Ac-Dc-Dc Converter Using Silicon Carbide Schottky Diode
of SiC Schottky diode in the chopper circuit. The maximum swing of the currents is almost identical in value.
However, the minimum peak is slightly greater for the SiC Schottky diode by merely 0.91 %. From this result,
eventually, the output power can be calculated and compared. The MOSFET’s Vds overshoot is higher in SiC
diode. This contradictory result makes the only drawback in the device. SiC has an overshoot voltage of 6 V,
Which is 6 % higher than Si that only 6.5 V. This is caused by the charge removal response time in SiC where t d
is higher as previously shown in Fig. 3(a). Nevertheless, this results may not affect the improvement in the
circuit because the variation in load circuit design may also cause to this drawback. Even though the MOSFET
peak voltage overshoot in SiC chopper circuit is higher, the corresponding output power, P out of the circuit is
slightly lower.When MOSFET is turned off, load current will start to discharge through the diode and this
overshoots the current further in the negative region before it goes back to zero mainly due to the removal of
large amount of charges in the diode. The faster the removal of charge or the smaller of charge appeared in the
diode will make the transient response faster.
Table 3 Current and Output Power
RLoad = 55 Ω
SiC
Schottky
IRLoad,min
8 mA
IRLoad,max
285 mA
IRLoad,avg
148 mA
Pout
2.664W
Refferences
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
I. Batarseh “Power Electronic Circuits”, University of Central Florida, John Wiley &Sons, Inc., 2004.
A.P. Malvino, “Transistor Circuit Approximation”, McGraw-Hill, Inc., 3rd Ed., 2007
F. Mohammed, M.F. Bain, F.H. Ruddell, D. Linton, H.S. Gamble and V.F. Fusco, “A Novel Silicon Schottky Diode for NLTL
Applications”, IEEE Transactions on Electron Devices, vol. 52, iss. 7, pp. 1384-1391, July. 2005.
B. Ozpineci and L.M. Tolbert, “Characterization of SiC Schottky Diodes at DifferentTemperatures”, IEEE Power Electronics
Letters, vol. 1, no. 2, pp. 54-57, Jun. 2003.
M.S. Chinthavali, B. Ozpineci and L.Mci and L.M. Tolbert, “Temperature-Dependent Characterization of SiC Power Electronic
Devices”, IEEE Power Electronic in Transportations, pp. 43-47, Oct. 2004.
M.J. Kearney, M.J. Kelly, A. Condie and L. Dale, “Temperature Dependent Barrier Heights in Bulk Unipolar Diodes Leading to
Improved Temperature Stable Performance”, IEEE Electronic Letters, vol. 26, iss. 10, pp. 671-672, May 1990.
B.J. Baliga, “Power Semiconductor Device Figure of Merit for High-FrequencyApplications”, IEEE Electron Device Letters, vol.
10, iss. 10, pp. 455-457, Oct. 1989.
Purdue University Nanoscale Center, Wide Bandgap Semiconductor Devices.
VI.
Acknowledgment
The authors are grateful for the support and encouragement of Dr. M. N. Channabasappa, Director, and
Dr. Shivkumaraiah, Principal of Siddaganga Institute of Technology, Tumkur
Biographies
Y. S. Ravikumar received B.E degree from Bangalore Universityin1981 and M.S. degree
from BITS, Pilani in 1985. Since 1983 he had been with Electronics and Communication
Engineering Department, SIT, Tumkur. At present he is an Associate Professor in the
Department of Telecommunication Engineering, SIT, Tumkur. His research interests are in
SiC device and electronics technology, and
power electronics devices. He has two
international conference papers and three publications to his credit.
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